1. Field of the Invention
The present invention relates to an image forming apparatus, and more particularly, to an image forming apparatus capable of determining whether the printing medium cassette is properly mounted in the image forming apparatus and whether the printing medium is present in the cassette.
2. Description of the Related Art
An image forming apparatus refers to an apparatus that prints an image on a printing medium according to an image signal. An image forming apparatus may generally be classified as a printer, a copying machine, a fax machine, a multi-function printer, which has multiple functions of printing, scanning, copying and faxing, or the like. In order to supply a printing medium (Hereinafter, referred to as “paper” for convenience. However, a printing medium is not limited to paper), an image forming apparatus comprises a paper feeding unit. A paper feeding unit generally includes a paper sensing device to sense the presence of the paper.
FIGS. 1a to 1c are views showing a conventional paper feeding unit having a paper sensing device. As shown in FIGS. 1a and 1b, a conventional paper feeding unit 1 includes a cassette 2 having a paper loading plate 2a, and a paper sensing device 3 to sense whether paper is present on the paper loading plate 2a. 
The paper sensing device 3 includes an actuator 4, a sensor 5 and an operation hole 6. The actuator 4 has a pivot shaft 4a, and an operation arm 4b and a sensing arm 4c, which are connected to the pivot shaft 4a. The operation arm 4b extends toward the paper loading plate 2a, and the sensing arm 4c extends toward the sensor 5.
The operation arm 4b is provided with a paper contact part 7 at an end portion thereof, which contacts with paper P loaded on the paper loading plate 2a. The operation hole 6 is formed on the paper loading plate 2a, so as to receive the paper contact part 7 of the operation arm 4b. When no paper is on the paper loading plate 2a, the paper contact part 7 of the operation arm 4b is received in the operation hole 6.
When the paper P is on the paper loading plate 2a, as shown in FIG. 1a, the paper P interferes with the contact part 7, and accordingly the actuator 4 is pivoted upward, so that the sensing arm 4c is positioned away from the sensing region of the sensor 5. A control unit of the image forming apparatus determines that the paper is on the paper loading plate 2a, and performs printing operation according to a print command.
On the other hand, when no paper is on the paper loading plate 2a, as shown in FIG. 1b, the paper contact part 7 of the operation arm 4b is received in the operation hole 6, and an end portion of the sensing arm 4c is positioned in the sensing region of the sensor 5. If the sensor 5 senses the sensing arm 4c, the control unit of the image forming apparatus determines that no paper is on the paper loading plate 2a. 
However, in the above conventional paper feeding unit, even when the cassette is not properly mounted in a main body, printing operation may be carried out, which may create a problem of improper paper supply or paper jam. For instance, as shown in FIG. 1c, even when a user inadvertently did not push the cassette 2 all the way in the main body into the proper cassette mounting position, the operation arm 4b is interfered with by the paper P loaded on the paper loading plate 2a, and accordingly the sensing arm 4c may be positioned away from the sensing region of the sensor 5. In such a case, notwithstanding the abnormal circumstances in which paper supply cannot be achieved correctly, the control unit of the image forming apparatus may determine that the apparatus is in a state capable of performing normal printing operation, and may attempt to perform a printing operation. Accordingly, a printing error may occur.
This problem may be solved by mounting an additional sensor capable of sensing proper mounting of the cassette. However, the installation of the additional sensor to the paper sensing device may make the internal structure of the image forming apparatus complicated, and may increase the manufacturing costs due to the increase in the number of components, thereby rendering the resulting product less competitive.
In addition, according to a recent trend of compactness of the apparatus, the main body may be formed smaller than the cassette. Typically, when mounting the cassette, a user puts the cassette in the main body until the front surface of the cassette is positioned on the same plane as the front surface of the main body. However, in a case where the main body has a smaller form factor than the cassette, though the cassette is perfectly mounted in the main body, a part of the cassette may protrude from the main body. Thus, a user cannot easily confirm visually whether the cassette is properly mounted in the main body, and thus is more likely to make the mistake of incorrectly mounting the cassette. In this regard, due to a current trend of compactness of the image forming apparatus, the impetus for providing a device capable of preventing a printing error by sensing whether the cassette is properly mounted may be especially high.